Hardened glass and method of making the same



Patented :Sept; 14;- 931 HARDENED: Gmssmamrnon or MAK- ud-me SAME FritzEckert, Beriim-Charlottenburg, Germany,

assignor to The American Securit Company,

WilmingtomD'eL, a corporation of Delaware No Drawing. Application 26,1935, Serial .No. 33,411. In Germany February 13, 1932 9 Claims;

'I'he'invention relates to the toughening or hardening of glass or glassobjects by heating the glass object almost to the softening'pointv(deformation point) andthen exposing it to a sudden adequate fall intemperature.

7 According to the oldest toughening or hardening process of De laBastie, glass objects heated almost to the softening point,'are chilleddirectly in an oil bath having a temperature of fill-250 10 C. Accordingto the later Siemens processes, the chilling is performed on flat glassby contact with cold metal plates or the like. When the cooling takesplace rapidly, stresses are caused and it is on the presence of thesestresses that the tough- 15 ening or hardening efiect obtained by theseprocesses is based. In order to produce well toughened or hardenedglass, a considerable and rapid fall in temperature must characterizethe cooling operation. On the other hand, this rapid fall in temperaturecaused a high percentage of breakage in the course of manufacture oftoughened glass.

In an attempt to avoid this, the tall in tempera ture was regulated byeither somewhat adjusting the length of the temperature range, or byregu- 25 lating the chilling speed byusing less abrupt cool ing means,for example, by using compressed air as the cooling means instead 01cold metal plates or the oil bath, the cooling in the lessabrupt methodbeing however still rapid-as compared 30 with the very slow coolingemployed in the manufacture of unhardened glass. 1

When the glass is greatly toughened or hardened, it is common knowledgethat apart from the desired increase in strength, the particular glass,

85 when broken, crumbles to small fragments or even to dust,'freeorsubstantially free from dangerous' splinters. It is also known thatsuch 'a great toughening or hardening can :only be attained it, incooling, the fall in-temperaturevis exploited to the utmost possibility,and there is ample uniformity in the distribution or the cooling means.p 7

As is known, in the cooling or glass two types of stresses arise, via,permanent and temporary ones.

The development of the permanent stresses,

which cause the anisotropic condition of the glass,

may he explained as follows: 1 r

By rapid cooling the surface of the glass is suddenly solidified, thecentral parts being still hot,

and consequently much diiated.' Whenthese. last become solidified andcooleddown they must have retained points of adherence-to the surface,and

consequently occupy a larger volume than that which agrees withthetemperatiu'e to which they .rpermanent stresses,

are reduced; the central molecules are, therefore, more distended thanusual, and exert a powerful, contracting force on the surrounding parts.At the instant when a part of the envelope or outer portion is broken,the molecules which had been 5 retained by it briskly contract, draw'inwith them all the others,"and thus determine a multitude of points ofrupture.

The temporary stresses are mainly caused by temperature differenceswhich are present in the 10 glass, while it is in a temperature range ofthe elastic-solid state of glass and'is in itseli isotropic, i. e., in alower temperature range than that in which the glass is conditioned forthe subsequent development of the permanent stresses therein.

It should be perhaps mentioned that, though the permanent stresses areoriginated by temperature differences arising in the glass so long asthe glass is partly in the plastic state, these permanent stresses areonly set up at a later stage of the cooling, whereas'the temporarystresses which are caused in a lower range of temperature are set upearlier than the permanent stresses. Y

The permanent stresses appear mainly in the 5 form of compression of thesurface layers and tension of the inner layer of the plate, whereas thetemporary stresses on the contrary appear mainly in the form of tensilestresses at individual parts of the surface layers. The invention isbased on recognition of the fact that in the course of the hithertoknown toughening methods both the temporary and the permanent stre weredeveloped in the glass but that the subsequent final hardened conditionofv the glass depends only upon the extent of the whereas the temporarystresses are not only useless as regards the degree of tougheningbut inpractice generally lead to breakage.

The recognition of thisiact is utilized for an improved method of glasshardening, wherein, according to the invention, the hardening procedureis so arranged as to obtain as large an amount as possible of permanentstresses in the glass, while at the same time avoiding the developmentof transient or temporary stresses aster as possible. Thus, inaccordance with the invention, the possibility is provided of obviatingthe disadvantages of the previous-hardening processes while at the soduction of unhardened glass.

10 uselessly increasing these dangerous temporary stresses, is thusavoided. The continued cooling of the glass until it assumes thetemperature of the surrounding atmosphere then takes place gradually inthe manner customary for the pro- To effect a more rapid interception ofthe chilling operation at the required moment, a brief heat impact mayalso be made to play on the surface of the plate, say by drawing theglass through a veil of flame. 20 The chilling procedure is therefore,according to the invention, confined to that temperature range which ismainly responsible for the conditioning of the glass for the subsequentdevelopment of the permanent stresses, whereas the 26 lower temperaturerange, within which temporary stresses arise during chilling, is nolonger used for chilling, a gradual cooling being ap-- plied insteadwithin this lower temperature range.

It has been found that this method results in 30 well or better hardenedproducts while at the same time the percentage of breakages occurringduring manufacture is materially decreased, if not entirely avoided.

Though I have tried above to give as full a' 35 theory of what happensduring the chilling and cooling of glass, I do not wish to be bound bythese explanations as my improved method of toughening or hardeningglass is practicable without making use of these theoretic ex-.

40 planations.

Thus for chilling according to the invention, a temperature range lyingbetween the softening temperature of the particular glass and atemperature at or near or close below its lower an- 45 nealingtemperature is used. The extent of this temperature range depends on theparticular nature of the glass and to a. certain degree also on thecircumstances under which'the glass obiect in' question has beenmanufactured, i. e., so

50 to speak on the history of the glass object. The smaller theavailable temperature range, the quicker must the chilling be, i. e.,the more effec tive must the chilling means be. In contradistinction tothis, it has been customary hitherto 55 to do the chilling over agreater temperature range and correspondingly slower. v

The thinner the glass is, the greater is the requisite range of the fallin temperature, or the more effective must the means be in order also to50 obtain complete hardening to the point of splinter-free crumbling inthe case of thin glass plates. If, from what is taught by thisinvention, the fall in temperature in the case of thin plates isarranged to lie in the range within which the 65 glass is conditionedfor the subsequent development of the permanent stresses therein, thenthis degree of hardening can also be obtained with thin plates, e. g.,plates of less than 5 mm. thickness.

7 For the purpose of indicating the speeds necessary for hardening, itmay be mentioned that, according to the types of glass to be handled,ap-. proximately the following chilling periods should be used, in thecourse of which the glass must 1 be chilled from the lower softeningtemperature.

compressed air. For intensification to the lower annealing temperature:for glass -8 mm. thick, chilling periods of 5-15 seconds, and for glass2-3 mm. thick, of 3-10 seconds will serve, the initial chilling beingcorrespondingly intensified, whereas hitherto chilling has ex- 5 tendedover substantially longer periods.

The invention is preferably employed in conjunction with those knownhardening processes where the chief cooling means is air, preferablypurposes, water, oil or steam can be added to the current of air. Fluidsor steam are added in a finely atomized form, for preference. Highlycondensed gases can be used as carriers, e. g1, highly condensedcarbonic acid.

It is advisable to use cooling means, whose cooling effect is fullyadjustable; in this way the cooling procedure can be supervised withsumcient accuracy to enable the total favorable temperaturerange to beemployed without it being overstepped. The regulation of the coolingeffect can for instance be performed by regulation of current velocityand/or of the pressure of the air or gas stream and/or by regulation ofthe quantity and size of the droplets of the additional means such aswater and oil.

According to the present invention the hardening process is so performedthat the production of temporary tensile stresses is excluded as far aspossible and the prescribed conditions for the production of permanenttensions are increased as far as possible. For this purpose the range ofthe sudden fall in temperature is put as high as possible, the upperlimit of this range being taken at or near the softening point and thelower limit by a point at or near the lower annealing temperature of theparticular glass. Within the available range the intensity of thechilling is regulated, for instance by the choice of suitable propertiesfor the cooling means. If the available temperature range is large,milder cooling means can be taken; if the temperature range of a certaintype of glass is small, a sharper cooling means must be used;furthermore the thinner the wall of the glass object to be hardened, thesharper must the cooling means be.

When carrying the new process into practical eflect, it is convenient tomake use of measures and devices customary in older hardening processesfor the purpose of chilling and cooling. so

The temperatures and chilling speeds to be used in hardening accordingto the present invention depend in each individual case on the nature ofthe glass and other circumstances presenting no dimculty to the personskilled in the art. Below,

several practical examples are given, without however, restricting theinvention to the data. cooling"means or constructional devices mentionedin theseexamples.

Example I A ground and polished glass plate, 25-35 cm. high, -125 cm.wide and 0,6 cm. thick, of a type of glass whose softening temperaturelies -at approximately 650 deg. C, is heated within .5 5-6 minutes in anoven temperature-of 720 deg;

mesa-040' 0,8 mm. blast-pipes-whichhave a distance 01 50 1 mm. from oneanother, and the 1 distance of the blast-pipes from the plate surfacesbeing about '75 mm.

Example 11 A glass plate, 60 cm. high, 90 cm. wide and 0,7 cm.thick,-which has been manufactured according to one or the known drawingprocesses and has been provided with round ground corners tinues to beheld in a vertical position is then exposed to the action of ventilatorair, which-is blown through funnels on both sides of the plate. Thechilling is performed within 10 to 15 seconds to say 450 deg. C., or in15 seconds to say 420 deg. 0., this temperature data referring to thesurfaces of the plates. the plate surfaces and funnel openings is about40 mm. and the ventilator air is fed under a pressure of a. 60 mm.column .of water.

Then follows a slow cooling to approximately deg. C. within 2 minutes bythrottling the air teed by 20%, the distance between the plate surfacesand funnel openings being increased to 550 mm.

The plate is then taken out of the ventilator aggregate and is slowlycooled within an hour by heatequalization down to the temperature of theroom.

- Example III A thin polished glass plate, 60cm. high,

cm. wide and 0,35 cm. thick, whose softening temperature lies atapproximately 640 deg. C. and its bottom annealing temperature atapproximately 460 deg. C., is heated in a vertical position within 3minutes to a temperature of approximately 590 deg. and is then chilledwithin 3 seconds to a temperature of approximately 430 deg. C. at thesurfaces of. the plate. A blastpipe aggregate is used as the coolingdevice in the manner mentioned in Example I, but the individualblast-pipes are constructed as swivelling pipes and so arranged that oneblast-pipe each operates on 100 cm? of the glass plate. Compressed airof 4 atm. and atomized water sucked in by the compressed air are used asthe cooling means. From the- 3rd to the 10th second the water is thencut off and of 3,5 atm. used.

Then a continual gradual cooling within 60 I minutes to deg. C.takesplace inside a cooling oven. The plate is then left and itstemperature becomes equal to the general temperature around it withinapproximately half an hour. I

Example 'I V A square plate, 40 cm. across and 8 mm. thick, 5 of glasshaving the same heating properties as The distance between onlycompressed air described in Exampleli, and it stays" there 12 seconds.Then for another 5seconds it is taken into the oven described in thebeginning, so

that its surface assumes a number of calories which does not howeversufllce to penetrate into the interior of the plate to so increase thetem perature there as-to amount to a substantial overstepp'ing of thelower annealing temperature.

What I claim is: 7 l. The method of hardening glass to develop.permanent stresses therein and to minimize the proximately thetemperature at which temporary stresses develop, at a slower rate. r

3. The method of hardening glass to develop permanent stresses thereinand to minimize the development of temporary stresses which consists inheating th e glass to approximately its softening point, rapidlytemperature approximately that of. its lower anand then cooling theglass chilling the glass to a healing temperature, applying heat brieflyto the glass, and then cooling the glass at a slower rate.

. softening point, rapidly chilling the glass to a temperatureapproximating the lower annealing point of the glass within a period oftime not exceeding fifteen seconds, discontinuing the rapid chilling,and thereafter cooling the glass at a slower rate.

6. The method of hardening glass not exceed-. ing eightmillimeters inthickness to produce permanent stresses therein and to minimize thedevelopment of temporary stresses which consists in heating the glass toa temperature, not

. exceeding 650 C;, rapidly chilling the glass to a surface temperaturenot less than 400 C. within-a period of time not exceeding fifteenseconds, and then cooling the glass at a slower rate.

'7. The method of hardening glass sheets or .other glass articles todevelop a maximum j amount of permanent stresses therein and to minimizethe development of temporary stresses which consists in'heating' theglass to approximately its softening point, rapidly chilling the heatedglass to approximately its lower annealing temperature, further chillingthe glass within a time approximately twice that of the initialchilling, thereafter gradually and slowly cooling 35 4. The method ofhardening glass sheets vor the glass over a substantial period of timeand then permitting the glass to assume the temperature of the normalsurrounding atmosphere. 8. A method of hardening glass having athickness not exceeding three: millimeters to produce permanent stressestherein and to minimize the development of temporary stresses whichconsists in heating the glass to approximately it's softening point,rapidly chilling the glass from its lower softening temperature to itslower annealing temperature within a period of time not exceeding tenseconds and thereafter cooling the glass at a slower rate.

9. The method of hardening glass not exceedtemperature within a periodof time not exceeding fifteen seconds, and thereafter cooling the glassat a slower rate.

. FRITZ ECKERT.

